Japanese Patent Laid-Open No. 11-22609 discloses technology (hereunder, referred to as “related art”) relating to switch control of a combustion method in an internal combustion engine that can switch the combustion method of the internal combustion engine from stratified charge combustion to homogeneous combustion, or from homogeneous combustion to stratified charge combustion. Since the air-fuel ratio in stratified charge combustion is leaner than the air-fuel ratio in homogeneous combustion, switch of the combustion method is accompanied by switch of the air-fuel ratio. As the method for switching the air-fuel ratio, there is known a method that gradually changes the air-fuel ratio so that a torque level difference does not occur. However, with this known method, a desired torque cannot be obtained although the torque level difference is reduced, and there arises the problem of causing degradation of emission because the air-fuel ratio which is not originally intended is used. The above described related art is proposed as the solution to this problem.
According to the above described related art, at the time of switching from homogeneous combustion to stratified charge combustion, only a target air fuel ratio is switched in a step manner before a target equivalence ratio is switched in a step manner. In more detail, an air amount is increased in advance by increasing only the target air amount in a step manner, and the target equivalence ratio is decreased in a step manner at a timing at which the actual air amount reaches the target air amount. That is to say, in a period in which the air amount is increased later than the target air amount, the target equivalence ratio before switching of the combustion method is maintained. However, if a fuel amount is determined based on the target equivalence ratio before switching of the combustion method, the fuel amount becomes excessively larger than an amount that is necessary to keep a torque constant. Therefore, in the above described related art, the excessive amount of the fuel amount is corrected by retardation of an ignition timing, whereby avoidance of increase in the torque at a time before switching of the combustion method is performed.
Incidentally, when the air-fuel ratio is switched from a theoretical air-fuel ratio to a lean air-fuel ratio in an internal combustion engine equipped with a variable valve timing mechanism that changes a valve timing of an intake valve, switching of a target valve timing from a valve timing corresponding to the theoretical air-fuel ratio to a valve timing corresponding to the lean air-fuel ratio is performed. Thereby, the target valve timing is switched so that a valve overlap amount decreases, in response to switching of the air-fuel ratio from the theoretical air-fuel ratio to the lean air-fuel ratio.
Here, when the target valve timing is switched at a timing for switching the target air-fuel ratio, in a case of performing control of switching the target air amount prior to the target air-fuel ratio, as in the aforementioned related art, a lean air-fuel ratio is realized during a switching operation by the variable valve timing mechanism, and therefore, a possibility of a misfire due to worsening of combustion coincidentally arises. Therefore, the timing for switching the target valve timing can be also considered to be set at a time point of switching of the target air amount. However, if switching of the target valve timing is performed at the time point of the switching of the target air amount, the valve overlap amount is decreased prior to switching of the air-fuel ratio. In this case, a possibility coincidentally arises, that increase in the air amount becomes slow, and a transitional period until the actual air amount increases to the target air amount after switching of the target air amount is prolonged. Since in the transitional period, the actual torque becomes larger than a requested torque, the requested torque is realized by retardation control of an ignition timing and the torque level difference is eliminated. However, when the transitional period until the target air amount is realized is prolonged due to the influence of the decrease in the above described valve overlap amount, worsening of combustion and worsening of fuel efficiency are feared. Further, if the retardation control of the ignition timing lasts for a long time period due to prolongation of the transitional period, an increase in the temperature of the exhaust system components becomes a problem. The increase in the temperature of the exhaust system components can be prevented by providing a limitation onto the retardation time period of the ignition timing, but an increase in torque due to an excessive fuel amount cannot be avoided.